Conventional high speed comparators typically receive a differential input signal at a differential pair of transistors biased by a reference current (e.g., a tail current). In response to the differential input signal, the transistors selectively pass the tail current to one or the other of a pair of diodes. The diode receiving the tail current turns on and the remaining diode turns off. The resulting voltages at the diodes typically feed a gain stage that provides a single ended output signal.
If the differential input signal does not rapidly change, the voltage at the diode in the off state may slowly decay to a very low voltage level well below the threshold voltage of the diode. As a result, if the comparator attempts to switch on the diode (e.g., in response to a change in the differential input signal), the diode voltage may be required to swing from a very low voltage up to its threshold voltage. In contrast, if the differential input signal does rapidly change, the voltage of the diode in the off state may not decay far below its threshold voltage.
The different voltage swings of the diodes associated with different data patterns in the differential input signal may cause conventional comparators to exhibit varying data propagation delays which appear as data dependent jitter (e.g., duty cycle distortion) in the single ended output signal. Such jitter is particularly problematic for high speed comparators. As a result, there is a need for an improved approach to comparator design that reduces data dependent jitter associated with conventional comparators.
In one embodiment of the invention, a circuit such as a comparator circuit includes a differential stage adapted to receive a differential input signal and first and second diodes coupled to the differential stage. The first and second diodes are adapted to selectively switch on and off to provide a differential output signal at first and second differential output nodes in response to the differential input signal. The circuit may include an output stage coupled to the first and second diodes at the first and second differential output nodes, with the output stage adapted to convert the differential output signal to a single ended output signal. The circuit may also include a current source adapted to selectively provide a reference current to the first or second diode in an off state to reduce voltage swing of the first or second diode between the off and an on state
The scope of the invention is defined by the claims, which are incorporated into this section by reference. A more complete understanding of embodiments of the invention will be afforded to those skilled in the art, as well as a realization of additional advantages thereof, by a consideration of the following detailed description of one or more embodiments. Reference will be made to the appended sheets of drawings that will first be described briefly.
Embodiments of the invention and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures.